The present invention relates to rolling bearings for use in aircraft, food processing machines and other applications where high corrosion resistance is required. The invention particularly concerns efforts to upgrade the performance of such rolling bearings.
Rolling bearings are usually made of steel materials equivalent to SUJ2 (bearing steel) or SCR420 (case hardening steel) but if they are used in the presence of water or under humid conditions or in other corrosive environments, they will experience premature rust formation and become no longer suitable for subsequent use.
Under the circumstances, SUS440C and other martensite-base steels have been substituted as high chromium bearing stainless steels that have not only high corrosion resistance but also the hardness of HRC 58 and above which is required for bearings.
Rolling bearings made of stainless steels are in most cases used in severer conditions than those experienced by rolling bearings made of plain steels and particularly in the presence of water or under moist conditions or in other corrosive environments, wear resistance with the potential for poor lubrication being also taken into account is an extremely important factor, to say nothing of corrosion resistance. The carbon in steels is an indispensable element for providing increased hardness by the strengthening of martensite and the formation of carbides contributes to a further improvement in wear resistance.
With aircraft bearings which are to be used at high temperatures and speeds, high core strength under hoop stress is required in addition to high corrosion resistance and fatigue strength.
The martensite-base high chromium stainless steels typified by SUS440C have a problem in that if the carbon level increases, the high C and Cr content of the steel causes many coarse (&gt;10 .mu.m) eutectic carbides to form, thereby deteriorating not only the steel's corrosion resistance, rolling life and toughness but also its forgeability and machinability.
If simply hardened, the conventional martensite-base stainless steels have large amounts of retained austenite, so they are usually subjected to a subzero treatment after the hardening operation. In the case of aircraft bearings which are to be used in comparatively high temperatures or with which dimensional stability is particularly important, the subzero treatment is occasionally followed by tempering at elevated temperatures of 400.degree. to 600.degree. C. However, if the conventional martensite-base stainless steels are tempered at 400.degree. to 600.degree. C., their hardness will drop to HRC 55 to 57 or even below, thereby causing deterioration in fatigue strength and wear resistance.
Another problem with the conventional high carbon martensite-base stainless steels is that the secondary precipitation of Cr carbides occurs during tempering and that if tempering is done at high temperatures, the corrosion resistance of the steels will drop markedly.
With a view to solving the aforementioned problems, Unexamined Published Japanese Patent Application Sho 61-163244 proposed that the C and Cr content be reduced to retard the formation of eutectic carbides, thereby providing improved fatigue strength and toughness without sacrificing the wear resistance. However, the proposed attempt is unsatisfactory in the improvements of resistance and to wear corrosion and, what is more, no consideration is given to the dimensional stability problem resulting from the retained austenite and the effects on fatigue strength, corrosion resistance and wear resistance that may be imposed by the high-temperature tempering which is performed in order to decompose the retained austenite.
According to Unexamined Published Japanese Patent Application Hei 1-205063, there is provided a wear-resistant stainless steel part that is improved in core toughness by limiting the carbon level to 0.6 wt % and below and which has been carburized to impart high surface hardness. The part, however, is not satisfactorily improved in corrosion resistance.
Speaking of bearings that are specifically intended for use in aircraft jet engines and gas turbines, they are subjected to high temperatures and speeds during service as already mentioned and, hence, require not only high core strength under hoop stress but also sufficient hardness at elevated temperatures. Conventionally, these bearings are made of semi-high-speed materials such as AISIM50 and M5ONiL; however, these materials are so low in corrosion resistance that they tend to experience premature rust formation due to salt damage in unfavorable locations such as airports adjoining the sea. Since safety is of utmost importance to aircraft bearings, their life may be recognized to have ended no matter how limited the rust formation is. Therefore, from a corrosion resistance viewpoint, aircraft bearings are preferably made of stainless steels materials.
However, the conventional stainless steel materials such as SUS440C do not have satisfactory hardness under elevated temperatures and hence are not suitable for use as bearing materials in aircraft engines. A further problem with the stated type of high-carbon martensite-base stainless steels is that if they are tempered at high temperature, the chromium in the matrix will precipitate as carbides, thereby damaging its corrosion resistance considerably.